The present invention relates generally to Sagnac ring gyroscopes and more particularly to a Sagnac ring gyroscope with an optical coupler suitable for fabrication by integrated optics techniques.
The Sagnac ring interferometer, which employs a long single-mode fiber-optic loop, has shown promise as a passive gyroscope for navigational purposes. The properties of Sagnac fiber-optic gyroscopes are described in an article by Rashleigh and Burns entitled "Dual-Input Fiber-Optic Gyroscope", Optics Letters, Vol. 5, No. 11, p. 482, Nov. 1980. Typical Sagnac fiber-optic gyroscopes include means for introducing light beams into both ends of a fiber-optic loop and means for measuring the relative phase shift of the light beams exiting the fiber-optic loop in order to determine the rate of rotation in the plane of the fiber-optic loop. An optical coupler is utilized to introduce an incoming light beam into the interferometer, to split the incoming light beam into the counter-propagating beams in the fiber-optic loop and to cause interference between the light beams exiting the fiber-optic loop.
A typical gyroscope operates at maximum sensitivity, or quadrature, for small rates of rotation when the output beams of the optical coupler of the gyroscope have equal intensity at zero rotation rate. Prior art couplers having two input waveguides and two output waveguides, (2.times.2), do not operate at quadrature unless a non-reciprocal .pi./2 phase shift is induced into the optical path of the counter-propagating light beams. The means needed to induce the phase non-reciprocal .pi./2 phase shift increase the complexity and susceptiblity to noise of prior art gyroscopes using (2.times.2) couplers.
A (3.times.3) coupler as disclosed, for example, in U.S. patent application No. 320,999 by Sheem operates at quadrature for small rates of rotation but the increased separation of the optical paths providing interference between the light beams exiting the fiber-optic loop decreases the coupling between the beams, requires a longer coupling length and lowers sensitivity. Attempts to arrange the optical paths in non-planar configurations to increase coupling destroys the symmetry required for integrated optics fabrication.